Kelvin force microscopy of polymer and small molecule thin-film transistors

Abstract

By their nature, scanning probe microscopy (SPM) measurements are ideally suited to the study of organic thin-film transistors (TFTs). In Chapter 3 surface potential measurements are made on TFTs made from the polymer pBTTT. The modification of the surface morphology by annealing into the liquid crystalline phase is examined and the separation between crystalline layers is found to be in agreement with the literature.

Kelvin force gradient microscopy (KFGM) measurements provide a very detailed picture of the local potential within the channel of the TFT, allowing for the separation of the device and film mobilities in both the linear and saturation regimes. Once the effects of the contacts are removed, the linear mobility is found to be four times higher than the saturation mobility, although the model that best fits the data assumes a constant mobility. Short channel effects are studied in saturation and an upper bound placed on the magnitude of the channel-length modulation. The contacts are also studied in more detail and it is found that a broad region of the pBTTT film near the contacts shows increased resistance, modulated by the gate voltage.

In Chapter 4, KFGM is applied to TFTs produces with zone-cast TIPS pentacene as the active material. The seemingly uniform linear crystallites are found to exhibit a wide range of different behaviours, resulting in device characteristics based on statistical averages of many crystallites. The AFM is used to define a single-crystal device and this is found to contain two distinct regions of widely divergent mobility. The highest mobility regions imply that the maximum theoretical mobility is much higher than is achieved with zone-casting. Greater control over the crystallisation of the film could result in significant increases in device performance. Scanned gate microscopy measurements are also performed and susceptibility to local gating is linked to the conductivity of the film.

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